Stabilization of photographic bleach powders containing an alkali metal ferricyanide



tent

Lawrence G. Welliver, Binghamton, N.Y., 'assignor to General Aniline &Film Corporation, New York, N .Y., a corporation of Delaware No Drawing.Filed May 15, 1957, Ser. No. 659,213

Claims. (Cl. 96-60) The present invention relates to single powderphotographic bleach mixtures containing an alkali metal ferricyanide asthe oxidizing agent, and more particularly to the stabilization of suchbleach powders.

Most photographic bleach solutions which are used for the processing ofcolor film contain in aqueous solution an alkali metal fern'cyanide suchas lithium, potassium or sodium terricyanide as the oxidizing agentwhich converts the metallic silver formed during development into asilver salt which is soluble in silver halide solvents such as hypo.Typically, such a bleach solution contains, in addition to theferricyanide, a halogenizing or converting agent preferably an alkalimetal chloride or an alkali metal bromide which converts the silverferricyanide initially formed in the bleaching reaction into the lesssoluble silver chloride or silver bromide and thus impels the bleachreaction to completion.

It is essential that the bleach reaction be carried out at apre-selected pH and consequently, the ferricyanide bleach solutionscontain a bufiering agent which maintains the desired pH even thoughsmall amounts of acids or alkali may be carried into the bleach by thefilm. The pH of the bleach solutions falls between a range of from 3 to9, a pH between 4' and 6 being generally preferred. Among the morefrequently used ingredients of such buffer mixtures are alkali metalphosphates such as disodium and monosodium phosphate, sodium citrate,sodium bisulfate, citric acid or phthalic acid. Some of the ingredientsof the butter mixture act also as corrosion inhibitors and retard thecorrosive action of the ferricyanide bleach on metallic developingtanks, trays and the like.

For reasons of economy and convenience, the alkali metal ferricyanidebleaches are usually packaged and sold as dry powder mixtures. Normally,the stability of dry ferricyanide bleach mixtures leaves much to'bedesired. However, their stability is further drastically curtailed bythe presence of other ingredients which are added to ensure thatthebleach possesses certain desirable photographic characteristics.

For instance, a metal ferrocyanide is included which gives a temperingaction to a freshly mixed bleach solution in order to simulate theperformance of a partially used bleach. The rate of bleaching depends onthe 7 oxidation reduction potential which in turnis a function of pH andthe relative"ferrocyanide-fenicyanide concentration. No such oxidationreduction potential can be established so long as ferricyanide only isin the fresh solution, and no ferrocyanide has been formed as a resultof the bleaching action; The desired tempering action is achieved byadding to a fresh bleach solution specified amounts of ferrocyanide thusestablishing an oxidation reduction potential which otherwise would beset up only after part of the ferricyanide had been reduced toferrocyanide.

In such powder admixtures, the ferricyanide has an even greater tendencyto cause caking and discoloration. It is believed that thisdiscoloration is caused by the de- Patented July 5, 1960 2 compositionof ferricyanide yielding ferrocyanide and iron salts leading in alllikelihood to the formation of Prussian Blue or Turnbulls Blue. As aby-product, a gas is formed which creates a pressure'build-up inhermetically sealed packages. This gas has been identified as hydrogencyanide which is highly toxic. 'Thus, the decomposition of ferricyanidebleach powders creates a serious health hazard. Other ingredients whichafiect adversely the stability of the ferricyanide bleaching powder arealkali dichromates which are added to bleach solutions to inhibit thecorrosion of processing equipment. Ferricyanide bleaches containingdichromates also cake and discolor with the formation of hydrogencyanide.

I have discovered that boric anhydride is an etiective stabilizer forphotographic bleaches particularly those which contain, in addition tothe alkali metal ferricyanide, a small amount of alkali ferrocyanide oralkali dichromate. Photogaphic bleach mixtures containing alkali metalferricyanide as the oxidizing agent and stabilized with boric anhydrideconstitute the purposes and objects of the present invention. V

The photographic bleach powders which are contentplated herein containas their essential ingredients an alkali metal ferricyanide and boricanhydride, together with other bleach adjuvants including buir'ermixtures, alkali metal halides, particularly alkali metal bromidesandalkali metal ferrocyanides or alkali metal dichromates; Photographicbleach powders which we have found stable during storage forconsiderable periods of time may be compounded in the followingproportions, it being understood that all parts are by weight and'thatthe resulting dry powder is to be used to make a liter of bleachsolution:

In compounding the above bleach powder, one may proceed by mixing theboric anhydride to the alkali metal ferricyanide and adding the otheringredients later; However, if preferred, all the ingredients may beadmixed simultaneously. One of the most important advantages of boricanhydride lies in the fact that its stabilizing action is not impairedby changes in alkalinity and acidlty.

In spite of the anhydrous nature of the boric anhydride, it issurprising to note that the stabilizing action is not based on adehydrating action and is not influenced by the selection of bleachingredients with ditferent amounts of water of crystallization. While itis desirable to use a grade of boric anhydride which is completely freeof water, it must be kept in mind that small amounts of moisture areabsorbed by the anhydride during storage and before the mixing of thebleach. However, these small amounts of water absorbed which usually donot exceed 2 percent, but which may go up as high as 5 percent based onthe weight of boric anhydride, do not interfere with the stabilizingaction on the bleach powder.

The following examples will serve to illustrate my invention but are notrestricted thereto. The quantity of the bleach mixtures represented bythe following formulae is to be dissolved in sufi'icient water to giveone liter of bleach solution.

Example I temperature for 6 months. I

' powderinixture from which the boric anhydride was a seam. bromide 15.0

Disodium'phosphate 11.0 Citric acid 8.0 J Boric anhydride 10,0

@Th' tier mixture; whicheontained potassium ferro- 'd not deteriorateafter incubation at 120 F. for} 72 .=ho1 irs or storage for 3 monthsatroomtemperaidentieal' p'owd'er mixture from which the boric anhydridewas omitted, was badlycaked and discolored to afgieeprblue colorwiththe-formation of highly toxic hydrogen cyanide rgas. 7

" axmp'lg ll;

{This powder mixture did not deteriorate during incuba-v tionfior bile'week at 110 F. or after storage at room. 7

By comparison, a similar otiiittedwas badlycaked and discolored to alight blue color the formation 'of highly toxic hydrogen-cyanide Thispowder mixture which contained potassium dichromate did not deteriorateduring incubation for one week at 120 F.'or storage at'room temperaturev(74 F.)

for "six months. A comparison mixture from which the boric anhydride hadbeen omitted was Submitted to the same storage conditions. 'It caked anddiscolored to a deep blue color with the formation of highly toxichydrog'en cyanide gas.

7 Example IV This powder mixture did not deteriorate after storage for"6 months at an average room temperature of 75 F.

identical pbwder mixture from which the boric anhydrid'e omitted wasbadly caked and discolored to a Hee blue colof after new 'da'ys.

V l h is powder mixture 'was kept for 6 months at roomtemperaturewithout deterioration. "A mixture without boric anhydridecaked badly' after a few days and discolored to a deep blue 'color withthe-formation of hydrogen eyaziide gas.

Example V1 r w Grams Potasium ferricyanide 75.0 Potassium ferrocyamde 5.0 Sodium bromide. '2;. 1020 weightofan"alkalizmetal rlichroniate,theflpartsfbeiag from 2 to 12 parts by weight of alkali metalferrocyanide,-

. 4 Monosidium phosphate 10.0 Phthalic acid ;'i 4.0

Boric anhydride V '12.0

.This powder mixture did not deteriorate when stored, for 6 months atroom temperature. A'type mixture from g which the boric anhydride hadbeen omitted became 7 colored and eaked badly aftera few days. C 1 gVarious modifications such-'21s A substituting. iequivalent materialsand varying the.proportions bf materialsarsed will be obvious to personsskilled in, the art. 7 Therefore,

the scope of the -inyefitiz jg g hmlvtf-eapsmeriylby g 1 Vappendedclaims; H

I claim:

1; A stabl'esingleawda aiihotegraphifblekh ture containing as itsessential eomponents .from 30 to,

parts by weight'of an alkali metal ferricyanidaffrom 2 to 20 parts byweight of an alkali metal bromide, from 4 to 25mm ofboric anhydride:an'd'ifro'm 2 to -12parts by weight of an alkali metalferrocyanide, theparts being based on the total 'wei ht of the powder. I '2. A stablesingle powder photographic bleach mixture containing as its-essentialcomponents from ,30 to 120' parts by weight of :an alkali metalferricyanide, fromiqte i 20 parts by weightof an alkali metal bromide.'fro'm'4'gto V 25 "parts of borio 'anhydride andsfrom .2 't'o 12 partsbybased on the total weight of theipo a'der;

3. A stable single powder photographic .bieaeh mixture containing asits. essentialgingredients for the P1761131 ration of one liter ofbleach asolution fromi30 to 12.0

parts 5 by weight of'alkali m'etal ferricyan'ide; from 2 to 20 parts byweight (if 'al'kalim'etal bromideffrom 5 to 30 a parts byweight"ofza'bufier .mixture comprising 'a salt selected from the:gi'oupfconsisting of alkali :metal plies phates and alkali metalcitrates, "and an acid selected from thegroup consisting-ofcitr-ic andphthalic acid; and

from 4 to 25 parts byweightiof boric anhydride, the parts being based onthe total weight of the powder. 4. A stable single powder photographicbleach ture containing as its essential ingredient-stolthe preparationof onelliter of bleach solution: from 30 to 120 parts by weight ofalkalimetal ferricyanitle; from 2 to 20. parts 'by weight of alkalimetal bromide; from 5 to 30 parts by weight of a butter-mixturecomprising a salt selected from the group-consisting :of alkali metalphosphates and alkali :metal 'citr'ates, and an acid :selected from the.group 'consistingzof citric and phthalic acid; g

from 4 to 25 parts by weight of b'oric a'nhyd'r ide; and

the parts being based on .the total weight ofthepowder. .5. A stablesinglepowderphotographic bleach mixture containing as its essentialingredients for the preparation '7 of one-liter 'of bleach solution:from 30:10 120 parts by weight of alkali metal ferricyanideyfrom 2 to20- parts by weight -of- -'alkali-metal bromide;ffromi to 30 parts :byweight of a buffer mixtnrec0mptising a-salt selected from the groupconsisting of alkali'met'al plies-. phates and alkali metal citrat'egand an acid selected from the group consisting eof-Zcitric' andfphthalicacid;-

from 4 to 25.p'a'rts by 'weight'ofboric anhydride; and from 2 to 12pa'rtsbyweight of alkalimetaldichromate, the parts being basedonthetotalweight ofithelpowder; 7

'6. A stable'sin'gleipowder photographic bleach mixture comprising analkali metal fenicyanidq an alkali m'etal ferrocyanide, ania'lkalianetalbromide and 'a-stabiliiing' amount of boric 'anhydride inaS quantitysufficient to prevent 'cakingfand disodlorationbf said bleach mixture; 1

A stable single powder phetographic bleach mixture 7 comprising "anL'alkali met'al ferricyanide, an' alkali metal bromide, a butfer mixmreco n rising amemberselected from the group consisting of '=alkali metalphosphates and alkali metal citrates and a member selected amine I groupof citric acid and phthalieacid, :and a stabilizing amount of bo'ricanhydride in fa quantitysufiieient .to prevent calsiirgandaltscblorzition otiatidhkheh:mixture;

8. A stable photographic bleach mixture having the followingcomposition:

Grams Potassium fern'cyanide 50.0 Potassium ferrocyanide 5.0 Sodiumbromide 15.0 Disodium phosphate 11.0 Citric acid 8.0 Boric anhydride10.0

9. A stable photographic bleach mixture having the followingcomposition:

10. A stable photographic bleach mixture having the followingcomposition:

Grams Potassium fenricyanide 50.0 Potassium ferrocyanide 5.0 Sodiumbromide 15.0 Monosodium phosphate 10.0 Phthalic acid 2.0 Boric anhydride10.0

References Cited in the file of this patent UNITED STATES PATENTS1,656,235 Plagwitz Ian. 17, 1928 2,606,118 Rogers Aug. 5, 1952 2,611,699Zappert Sept. 23, 1952 2,650,878 Boyer et a1. Sept. 1, 1953 2,843,482Kridel July 15, 1958

1. A STABLE SINGLE POWDER PHOTOGRAPHIC BLEACH MIXTURE CONTAINING AS ITSESSENTIAL COMPONENTS FROM 30 TO 120 PARTS BY WEIGHT OF AN ALKALI METALFERRICYANIDE, FROM 2 TO 20 PARTS BY WEIGHT OF AN ALKALI METAL BROMIDE,FROM 4 TO 25 PARTS OF BORIC ANHYDRIDE AND FROM 2 TO 12 PARTS BY WEIGHTOF AN ALKALI METAL FERROCYANIDE, THE PARTS BEING BASED ON THE TOTALWEIGHT OF THE POWDER.